Aircraft operating conditions provide unique challenges for the design of aircraft fire suppression systems. For example, aircraft fire suppression systems must work at a wide range of temperatures. The limits of these temperature ranges may extend from +105° C. when the aircraft is on the tarmac on a hot day, to as low as −55° C. when the aircraft is at high altitudes.
For more than 50 years Halon 1301 has been the agent of choice for aircraft engine and APU fire suppression applications. Halon 1301 has specific desirable properties that facilitate distribution of the agent throughout all areas of the fire zone. The low boiling point of Halon 1301 and the material's ability to freely vaporize at each outlet or discharge location in the distribution system, even in harsh environments, are desirable physical properties. Halon 1301 is typically stored in a pressurized bottle, using nitrogen as the pressurizing gas. Upon discharge, the agent freely vaporizes, changing state from a liquid phase to a gas phase and completely mixing with air.
Aircraft fire suppression systems are designed based on the agent weight, traditionally Halon 1301, required to achieve a specific minimum agent concentration in the fire zone immediately after agent discharge. In order for a fire suppression system to obtain aircraft certification, the system must demonstrate the ability to achieve a minimum specified agent concentration within the subject fire zone. A certification test is typically performed by discharging the fire extinguisher bottle and measuring the agent concentration in the fire zone during aircraft operation, either in flight or on the ground.
Agent concentration is measured by sampling the air in the fire zone and measuring the concentration of the Halon vapor within the sampled air. During a typical aircraft fire suppression system test, approximately twelve (12) probes of small tubing are placed at designated locations within the fire zone. The opposite end of each probe is connected to the measuring instrument. Different measuring instruments or sensors may be used depending on the particular fire suppression agent that is being used. The Halonyzer is an example of a well-known measuring instrument. A pump in the measuring instrument is used to draw a steady stream of air from the fire zone, through each probe, and into a sensor. Output from the sensor indicates the concentration of Halon gas relative to air. The relative measurement indication is converted to a volumetric concentration value.
The Halonyzer instrument and the 24-channel CME (concentration measurement instrument) are presently only capable of measuring the concentration of gas phase fire suppression agents relative to air. Halon concentrations within the sampled air may be measured using standard sample probe techniques because the Halon completely vaporizes in the air.
Environmental concerns over ozone depletion due to Halon 1301 have resulted in “green” activity to identify possible fire suppressing agent replacements for Halon 1301. Many of the agents that are potential “green” replacements for Halon 1301 have a higher boiling point than Halon 1301. Any potential agent that has a boiling point higher than the temperature in the fire zone when discharged will not freely vaporize, i.e. change state spontaneously from liquid to gas upon discharge. Upon discharge at low temperatures, many of the potential replacements for Halon 1301 exist as a “fog mist” because of their higher boiling points. The “fog mist” is a mixture of agent vapor (gas phase) and small liquid phase drops. An accurate and repeatable concentration measurement of these “fog mists” is not possible using current gas probe measurement techniques. The liquid phase drops do not mix homogeneously with air and the drops stick to the sides of the sample tubes due to surface tension. The cohesive nature of the very small droplets in the “fog mist” and adhesive nature of the drops onto current sample tubing are problematic. No probe design or instrument is currently available to perform accurate and repeatable post-discharge measurements for indicating real-time volumetric or mass concentration values of agents that exist in both the gas and liquid form.